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Ecology: Re-complexifying impoverished food webs
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The global-scale decline of animal biodiversity ('defaunation') represents one of the most alarming consequences of human impacts on the planet. The quantification of this extinction crisis has traditionally relied on the use of IUCN Red List conservation categories assigned to each assessed species. This approach reveals that a quarter of the world's animal species are currently threatened with extinction, and 1% have been declared extinct. However, extinctions are preceded by progressive population declines through time that leave demographic 'footprints' that can alert us about the trajectories of species towards extinction. Therefore, an exclusive focus on IUCN conservation categories, without consideration of dynamic population trends, may underestimate the true extent of the processes of ongoing extinctions across nature. In fact, emerging evidence (e.g. the Living Planet Report), reveals a widespread tendency for sustained demographic declines (an average 69% decline in population abundances) of species globally. Yet, animal species are not only declining. Many species worldwide exhibit stable populations, while others are even thriving. Here, using population trend data for >71,000 animal species spanning all five groups of vertebrates (mammals, birds, reptiles, amphibians and fishes) and insects, we provide a comprehensive global-scale assessment of the diversity of population trends across species undergoing not only declines, but also population stability and increases. We show a widespread global erosion of species, with 48% undergoing declines, while 49% and 3% of species currently remain stable or are increasing, respectively. Geographically, we reveal an intriguing pattern similar to that of threatened species, whereby declines tend to concentrate around tropical regions, whereas stability and increases show a tendency to expand towards temperate climates. Importantly, we find that for species currently classed by the IUCN Red List as 'non-threatened', 33% are declining. Critically, in contrast with previous mass extinction events, our assessment shows that the Anthropocene extinction crisis is undergoing a rapid biodiversity imbalance, with levels of declines (a symptom of extinction) greatly exceeding levels of increases (a symptom of ecological expansion and potentially of evolution) for all groups. Our study contributes a further signal indicating that global biodiversity is entering a mass extinction, with ecosystem heterogeneity and functioning, biodiversity persistence, and human well-being under increasing threat.
The application of a functional trait‐based approach to ecological restoration is receiving growing attention worldwide, but lack of knowledge on functional traits and how they link to ecosystem services imposes a major barrier to operationalize such approach. Synthesizing the existing knowledge on functional trait‐based restoration is thus a timely and important challenge. We systematically reviewed the literature to assess how ecosystem services are associated to functional traits across organisms, ecosystem types, and continents. We also assessed the existing trait‐based frameworks to target ecosystem services in restoration ecology. Then, we discussed future perspectives for the field, especially the challenges of applying trait‐based frameworks in megadiverse tropical ecosystems, which have ambitious restoration commitments. Most papers focused on plants (72%), terrestrial habitats (69%), and non‐tropical ecosystems (68%), and monitored ecosystem services and functional traits after restoration started rather than using them as previous targets. Only 12% of the papers targeted the restoration of both services and traits a priori, and 3.8% presented a clear trait‐based framework to target ecosystem services in restoration. The possibility of selecting alternative subsets of complementary species in their provisioning of ecosystem services should make functional restoration more feasible than traditional approaches in species‐rich tropical ecosystem. With this review and our critical insights on the perspectives of applying functional trait‐based restoration widely, we hope to assist broad‐scale restoration programs to obtain higher levels of benefits for nature and human well‐being per unit of area undergoing restoration, going beyond the area‐based approach that has dominated restoration commitments.
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Our planet is changing at paces never observed before. Species extinction is happening at faster rates than ever, greatly exceeding the five mass extinctions in the fossil record. Nevertheless, our lives are strongly based on services provided by ecosystems, thus the responses to global change of our natural heritage are of immediate concern. Understanding the relationship between complexity and stability of ecosystems is of key importance for the maintenance of the balance of human growth and the conservation of all the natural services that ecosystems provide. Mathematical network models can be used to simplify the vast complexity of the real world, to formally describe and investigate ecological phenomena, and to understand ecosystems propensity of returning to its functioning regime after a stress or a perturbation. The use of ecological-network models to study the relationship between complexity and stability of natural ecosystems is the focus of this review. The concept of ecological networks and their characteristics are first introduced, followed by central and occasionally contrasting definitions of complexity and stability. The literature on the relationship between complexity and stability in different types of models and in real ecosystems is then reviewed, highlighting the theoretical debate and the lack of consensual agreement. The summary of the importance of this line of research for the successful management and conservation of biodiversity and ecosystem services concludes the review.
Significance
The island continent of Australia harbors much of the world’s most distinctive biodiversity, but this review describes an extent of recent and ongoing loss of its mammal fauna that is exceptionally high and appreciably greater than previously recognized. The causes of loss are dissimilar to those responsible for most biodiversity decline elsewhere in the world.
Large carnivores face serious threats and are experiencing massive declines in their populations and geographic ranges around
the world. We highlight how these threats have affected the conservation status and ecological functioning of the 31 largest
mammalian carnivores on Earth. Consistent with theory, empirical studies increasingly show that large carnivores have substantial
effects on the structure and function of diverse ecosystems. Significant cascading trophic interactions, mediated by their
prey or sympatric mesopredators, arise when some of these carnivores are extirpated from or repatriated to ecosystems. Unexpected
effects of trophic cascades on various taxa and processes include changes to bird, mammal, invertebrate, and herpetofauna
abundance or richness; subsidies to scavengers; altered disease dynamics; carbon sequestration; modified stream morphology;
and crop damage. Promoting tolerance and coexistence with large carnivores is a crucial societal challenge that will ultimately
determine the fate of Earth’s largest carnivores and all that depends upon them, including humans.
The study of invasion ecology usually focuses on the negative impacts of alien species, while potential positive impacts are often overlooked. Understanding of biotic interactions may thus be skewed towards the negative, which could have important implications for ecological management and conservation. This article provides a comprehensive review of all types of impacts, both beneficial and detrimental, that can result from species translocation. An extensive review of literature on species introductions to terrestrial, freshwater and marine ecosystems and involving a wide range of taxa (including microorganisms, parasites, plants, insects, amphibians, reptiles, birds, mammals, fish and Crustacea) showed that, despite limited research into facilitative alien-native interactions, such interactions occur surprisingly frequently. Examples were found of introduced species acting as hosts, food sources, pollinators or seed dispersers for native species, as well as providing herbivory, predatory or parasite release. However, research showed that numerous negative interactions also occurred and combination impacts (when an alien benefits some natives but disadvantages others) were common. In many cases, the traditional view that biological invasions constitute a significant threat to native biota is both accurate and appropriate. Efforts to prevent translocation and control non-native species can be vital. However, the "native good, alien bad" maxim does not convey the complexity of invasion ecology: alien species do not axiomatically pose a threat to native biota. In order to move understanding of invasion ecology forward and to develop maximally-effective management strategies, facilitative alien-native interactions need to be added into the alien species debate.
Human involvement in food webs has been profound, bringing about enormous and disproportionate losses of large apex predators on land and in water. The losses have modified or even eliminated concatenations of indirect interactions propagating from predators to herbivores to plants, inter alia. Food webs are a synthesis of bottom-up energy and nutrient flow from plant producers to consumers and top-down regulation of producers by consumers. The trophic cascade is the simplest top-down interaction and accounts for a great deal of what is known about food webs. In three-link cascades, predators suppress herbivores, releasing plants. In longer cascades, predators can suppress smaller mesopredators, releasing their prey animals. Hunting, fishing, and whaling have brought parallel losses of large apex predators to food webs. Without apex predators, smaller mesopredators have often become superabundant, sometimes with unprecedented suppression of their prey, extinctions, and endangerment. Flourishing mesopredators also can reverse the web regulation and suppress apex predators that have become rare owing to hunting and fishing. This can prevent fisheries recovery and lead to persistent alternative ecosystem states. Although food-web modules of large animals are increasingly well understood, the parts of webs consisting of small inconspicuous organisms, such as mutualists and parasites, and webs in obscure places, such as in the soil, are much of the challenge of future research.
Human populations and their use of land have transformed most of the terrestrial biosphere into anthropogenic biomes (anthromes), causing a variety of novel ecological patterns and processes to emerge. To assess whether human populations and their use of land have directly altered the terrestrial biosphere sufficiently to indicate that the Earth system has entered a new geological epoch, spatially explicit global estimates of human populations and their use of land were analysed across the Holocene for their potential to induce irreversible novel transformation of the terrestrial biosphere. Human alteration of the terrestrial biosphere has been significant for more than 8000 years. However, only in the past century has the majority of the terrestrial biosphere been transformed into intensively used anthromes with predominantly novel anthropogenic ecological processes. At present, even were human populations to decline substantially or use of land become far more efficient, the current global extent, duration, type and intensity of human transformation of ecosystems have already irreversibly altered the terrestrial biosphere at levels sufficient to leave an unambiguous geological record differing substantially from that of the Holocene or any prior epoch. It remains to be seen whether the anthropogenic biosphere will be sustained and continue to evolve.
Since prehistory, humans have altered the composition of ecosystems by causing extinctions and introducing species. However, our understanding of how waves of species extinctions and introductions influence the structure and function of ecological networks through time remains piecemeal. Here, focusing on Australia, which has experienced many extinctions and introductions since the Late Pleistocene, we compared the functional trait composition of Late Pleistocene (130,00–115,000 years before present [ybp]), Holocene (11,700–3,000 ybp), and current Australian mammalian predator assemblages (≥70% vertebrate meat consumption; ≥1 kg adult body mass). We then constructed food webs for each period based on estimated prey body mass preferences. We found that introduced predators are functionally distinct from extinct Australian predators, but they rewire food webs toward a state that closely resembles the Late Pleistocene, prior to the megafauna extinctions. Both Late Pleistocene and current-day food webs consist of an apex predator and three smaller predators. This leads to food web networks with a similar total number of links, link densities, and compartmentalizations. However, this similarity depends on the presence of dingoes: in their absence, food webs become simplified and reminiscent of those following the Late Pleistocene extinctions. Our results suggest that recently established predators, even those implicated in species extinctions and declines, can restore complexity to food webs simplified by extinction.
Most terrestrial large mammals went extinct on different continents at the end of the Pleistocene, between 50,000 and 10,000 years ago. Besides the loss in species diversity and the truncation of body mass distributions, those extinctions were even more impactful to interaction diversity. Along with each extinction, dozens of ecological interactions were lost, reorganizing species interaction networks, which attained species-poor configurations with low functional redundancy. Extinctions of most large herbivores impacted energy flow and the rates of nutrient cycling, reconfiguring ecosystem-level networks. Because large mammals have high mobility, their loss also shortened seed-dispersal distance and reduced nutrient diffusivity, disrupting spatial networks. This review examines the recent advances in understanding how different types of ecological networks have been restructured by megafaunal extinctions and how this reorganization affected ecosystem functions. ▪ Megafaunal extinctions resulted in the loss of multiple ecological interactions in terrestrial systems. ▪ Interaction loss reshaped different types of ecological networks including food webs and spatial networks. ▪ The reorganization of ecological networks changed how terrestrial ecosystems are structured and function.
Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 52 is May 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Samuel Turvey and Jennifer Crees introduce the effects of humans on biodiversity.
While many recognise that rigid historical and compositional goals are inadequate in a world where climate and other global systems are undergoing unprecedented changes, others contend that promoting ecosystem services and functions encourages practices that can ultimately lower the bar of ecological management. These worries are foregrounded in discussions about 'novel ecosystems' (NEs), where some researchers and conservationists claim that NEs provide a license to trash nature as long as certain ecosystem services are provided. This criticism arises from what we call the 'anything goes problem' created by the release of historical conditions. After explaining the notion of NEs, we identify numerous substantive motivations for worrying about the 'anything goes problem' and then go on to show that the problem can be solved by correcting two mistaken assumptions. In short, we argue that the problem is a product of adopting an overly sparse functional perspective that assumes an unrealistically high degree of convergence in the trajectories of natural processes; our analysis illuminates why such assumptions are unwarranted. Further, we argue that adopting an appropriate ethical framework is essential to overcoming the 'anything goes problem', and suggest that a certain virtue-ethics conception of ecological management provides useful resources for framing and resolving the problem.
There is increasing evidence that restoration ecologists should be most concerned with restoring species interactions rather than species diversity per se [1]. Rewilding with taxon substitutes, the intentional introduction of exotic species to replace the ecosystem functions of recently extinct species, is one way to reverse ecosystem dysfunction following the loss of species interactions [2]. This is highly controversial [3], in part because of a lack of rigorous scientific studies [4]. Here we present the first empirical evidence of an in situ rewilding project undertaken as a hypothesis-driven ecosystem management option. On Ile aux Aigrettes, a 25-hectare island off Mauritius, the critically endangered large-fruited endemic ebony, Diospyros egrettarum (Ebenaceae), was seed-dispersal limited after the extinction of all native large-bodied frugivores, including giant tortoises. We introduced exotic Aldabra giant tortoises, Aldabrachelys gigantea, to disperse the ebony seeds. Not only did the tortoises ingest the large fruits and disperse substantial numbers of ebony seeds, but tortoise gut passage also improved seed germination, leading to the widespread, successful establishment of new ebony seedlings. Our results demonstrate that the introduction of these exotic frugivores is aiding the recovery of ebonies. We argue for more reversible rewilding experiments to investigate whether extinct species interactions can be restored.